1. Field of the Invention
The invention relates to a turning determination apparatus and a turning determination method for a vehicle, and more particularly to an apparatus that accurately determines whether a vehicle is turning, using wheel speeds.
2. Description of the Related Art
There is a turning determination apparatus that determines whether a vehicle is turning based on signals that indicate wheel speeds of right and left wheels. This apparatus may make an erroneous determination when the roughness level of the road surface on which the right wheel moves is different from the roughness level of the road surface on which the left wheel moves.
Japanese Patent Application Publication No. JP-A-2000-280882 describes a turning determination apparatus for a vehicle, which prevents such an erroneous determination. The turning determination apparatus for a vehicle includes left wheel-speed detection means, right wheel-speed detection means, turning state signal output means, turning determination means, and road-surface determination signal output means. The left wheel-speed detection means detects the wheel speed of the left wheel of a vehicle. The right wheel-speed detection means detects the wheel speed of the right wheel of the vehicle. The turning state signal output means detects the state of the vehicle based on a difference between the wheel speeds of the right and left wheels, which are detected by the right wheel-speed detection means and left wheel-speed detection means, and outputs a turning state signal that indicates the state of the vehicle. The turning determination means determines that the vehicle is turning when the value of the turning state signal output from the turning state signal output means exceeds a predetermined turning determination threshold value. The road-surface determination signal output means detects the roughness level of a road surface, and outputs a road-surface determination signal that indicates the roughness level of the road surface. In the turning determination apparatus for a vehicle, the predetermined turning determination threshold value, which is used by the turning determination means, is set to a value obtained by adding the value of the road-surface determination signal output from the road-surface determination signal output means to a reference threshold value.
In the turning determination apparatus for a vehicle, the turning determination threshold value, which is used by the turning determination means to determine whether the vehicle is turning, is set to the value obtained by adding the value of the road-surface determination signal output from the road-surface determination signal output means to the reference threshold value. Thus, when the roughness level of the road surface on which the right wheel moves is different from the roughness level of the road surface on which the left wheel moves, the predetermined turning determination threshold value is higher than the reference threshold value by the added value of the road-surface determination signal. This prevents an erroneous determination from being made due to the road surface condition. Also, when the turning determination apparatus is provided in a vehicle equipped with an anti-skid control apparatus that includes wheel-speed detection means as an essential element, it is not necessary to newly provide a sensor. This suppresses an increase in the cost.
In the above-described turning determination apparatus for a vehicle, the state of the vehicle is determined based on the difference between the wheel speeds of the right and left wheels. When the value of the turning state signal, which indicates the state of the vehicle, exceeds the turning determination threshold value, it is determined that the vehicle is turning. Also, the turning determination threshold value, which is used to determine whether the vehicle is turning, is corrected based on the roughness level of the road surface. Because the wheel speed greatly varies depending on the road surface condition and the slip state of the wheel, the wheel speed may greatly deviate from a normal wheel speed.
In the above-described turning determination apparatus for a vehicle, although the turning determination threshold value is corrected based on the roughness level of the road surface, it may be erroneously determined that the vehicle is turning if the detected wheel speed is abnormal due to the road surface condition or the slip state of the wheel. That is, if one of the right and left wheels slips on a low-μ road surface when the vehicle moves on a straight road, the wheel speed of the wheel that slips suddenly changes. This increases the difference between the wheel speeds of the right and left wheels. As a result, it is erroneously determined that the vehicle is turning, though the vehicle moves on the straight road.